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# Law of Inertia and Frames of Reference

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Chapter 2, Section 1 notes

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### Law of Inertia and Frames of Reference

1. 1. Good Morning! <ul><li>Today you will need: </li></ul><ul><ul><li>Your spiral and notebook paper </li></ul></ul><ul><ul><li>Pen or pencil </li></ul></ul><ul><li>Today we will: </li></ul><ul><ul><li>finish our notes </li></ul></ul><ul><ul><li>watch some videos </li></ul></ul><ul><ul><li>demonstrate our understandings by: </li></ul></ul><ul><ul><ul><li>writing a short paragraph </li></ul></ul></ul><ul><ul><ul><li>taking a five question clicker quiz </li></ul></ul></ul>
2. 3. What We Learned <ul><li>In the Investigate, you observed, measured, and compared the height of a ball on one side of the track to the recovered height on the other side of the track. </li></ul><ul><li>What did you find out? </li></ul>
3. 4. Galileo Galilei <ul><li>Galileo Galilei was an Italian physicist, mathematician, astronomer, and philosopher. He is often called the “Father of Science” because he introduced experimental science to the world. </li></ul><ul><li>He observed that a ball rolled down one ramp seemed to seek the same height when it rolled up another ramp. </li></ul>
4. 5. Galileo Galilei <ul><li>From his observations and investigations, Galileo theorized that a ball made of an extremely hard material and rolled on a smooth horizontal surface would roll forever. </li></ul><ul><li>Combined with his observation that objects at rest stay at rest, Galileo formed the first thinking about inertia . </li></ul>
5. 6. Inertia <ul><li>Inertia is the natural tendency of an object to resist changes in its motion. </li></ul><ul><li>Inertia is a physical property of matter. </li></ul><ul><li>The more mass an object has, the more inertia it has. </li></ul>Copy This
6. 7. Back to Galileo <ul><li>Galileo changed the way in which people viewed motion. Before Galileo, people thought that all moving objects would stop. </li></ul><ul><li>It ’s easy to understand why: when have you ever seen an object roll forever? </li></ul>
7. 8. Isaac Newton <ul><li>Isaac Newton was born in 1642, the year of Galileo ’s death. </li></ul><ul><li>Newton used Galileo ’s ideas about inertia to form his first law of motion. </li></ul>
8. 9. Law of Inertia <ul><li>In the absence of an unbalanced force, an object at rest remains at rest and an object in motion remains in motion with a constant speed in a straight line. </li></ul>Copy This
9. 10. Isaac Newton <ul><li>It might seem that Newton “stole” his law of inertia from Galileo – however, he always credited Galileo and others for their contributions to his thinking. </li></ul><ul><li>He was quoted as saying, “If I have seen farther than others, it is because I have stood on the shoulders of giants.” </li></ul>
10. 11. Mass – Inertia connection <ul><li>Newton explained that an object ’s mass is a measure of its inertia. </li></ul><ul><li>In other words, given objects with different masses moving at the same speed, the one with the greater mass has the greatest inertia. </li></ul>Copy This
11. 12. Mass – Inertia Connection <ul><li>Which of the following carts has the greatest inertia? </li></ul><ul><li>a) 1 kg moving at 5 m/s </li></ul><ul><li>b) 2 kg moving at 3 m/s </li></ul><ul><li>c) 3 kg moving at 1 m/s </li></ul><ul><li>d) 4 kg moving at 1 m/s </li></ul><ul><li>The correct answer is (d) - the cart with 4 kg mass because speed is not important when determining inertia </li></ul>
12. 13. The Law of Inertia <ul><li>The Law of Inertia is easy to say, and easy to remember but it ’s hard to believe. </li></ul><ul><li>The “objects at rest part” makes sense – </li></ul><ul><li>but the part about requiring a force to stop an object or change its motion – and that absent that force the object would continue moving forever – is something you will have to remind yourself of many, many times </li></ul>
13. 14. Running Start <ul><li>What does it mean to have a running start? </li></ul><ul><li>Think about your favorite sport…is there a “running start” in part of it? </li></ul><ul><li>Watch these two videos of athletes jumping </li></ul><ul><li>Broad Jump Long Jump </li></ul>
14. 15. A Running Start – What Do You Think? <ul><li>An excellent distance for the broad jump would be about 12 feet – roughly 3.5 meters. </li></ul><ul><li>An excellent distance for a long jump is more than twice that – about 8.5 meters. </li></ul><ul><li>Why can long jumpers travel so much further? </li></ul>
15. 16. Good Morning! <ul><li>Today we will: </li></ul><ul><ul><li>explore running starts and why they help athletes throw, jump, and kick farther </li></ul></ul><ul><li>PLEASE DO NOW: </li></ul><ul><ul><li>get a whiteboard, a marker for your table </li></ul></ul><ul><ul><li>get out your spiral and something to write with </li></ul></ul>
16. 17. Running Start <ul><li>The javelin throw is another good example of the effect of a running start </li></ul><ul><li>Watch this video of throwing a javelin </li></ul>
17. 18. Running Start <ul><li>Which object travels farther: a baseball thrown 20 m/s in the air for 1 second or a baseball thrown 30 m/s in the air for 1 second? </li></ul><ul><li>The faster a person or tool is moving at release (or contact), the farther the object being thrown (or struck) will travel. </li></ul><ul><li>(All other things – release angle, choice of club, size of tool - being equal) </li></ul>
18. 19. Newton ’s First Law and the Javelin <ul><li>At the moment of release, the speed of the javelin is the same as the speed of the hand throwing the javelin </li></ul><ul><li>If the athlete applies additional force to move his/her elbow and shoulder the speed of the javelin will be the sum of these speeds </li></ul><ul><li>Describe how a running start increases how far the javelin will travel. </li></ul>
19. 20. Law of Inertia and the Javelin <ul><li>When we talk about running starts and moving hands, we ’re not really talking about speed anymore – we actually mean velocity. </li></ul><ul><li>Why is this distinction important? </li></ul>
20. 21. Velocity is a Vector! <ul><li>Time for some review: </li></ul><ul><ul><li>Velocity has both speed AND direction. </li></ul></ul><ul><ul><li>Quantities with both speed and direction are called vectors. </li></ul></ul><ul><li>Since the speed of the javelin is the SUM of the different speeds (speed of body, speed of hand), we should be able to show how much the total speed is. </li></ul>
21. 22. Velocity of the Javelin <ul><li>V javelin = V hand + V elbow + V shoulder + V body </li></ul><ul><li>Sports and motion are inseparable. Sometimes the speed is constant in sport and sometimes, an unbalanced (net) force acts and the motion changes. </li></ul>Copy This
22. 23. Changing Motion <ul><li>What are some ways motion can change? </li></ul><ul><ul><li>speed up </li></ul></ul><ul><ul><li>slow down </li></ul></ul><ul><ul><li>change direction </li></ul></ul><ul><li>In other words, changing motion is acceleration. </li></ul><ul><li>Acceleration requires an unbalanced (NET) force . </li></ul>Copy This
23. 24. Quick Check <ul><li>If you were to participate in a softball throw for distance, why would you want to take a running start? </li></ul><ul><li>Use scientific language in your answer as well as a vector diagram. </li></ul>
24. 25. Frames of Reference <ul><li>Imagine a small toy cannon that always shoots a small ball forward at 7 m/s </li></ul><ul><li>If you place the toy cannon on a skateboard and give the skateboard a push resulting in 3 m/s of skateboard speed in the same direction that the toy cannon shoots, what is the ball ’s speed? </li></ul>
25. 26. Frame of Reference <ul><li>What if you are standing on the skateboard while it moves forward at 3 m/s – how fast do the cannon balls leave the toy? </li></ul>
26. 27. Frames of Reference <ul><li>So, which answer is correct? Are the toy cannon balls moving 10 m/s or 7 m/s? </li></ul><ul><li>Both answers are correct – depending on your frame of reference. </li></ul><ul><li>The toy cannon balls are moving 7 m/s relative to the skateboard. They are moving 10 m/s relative to the ground. </li></ul>
27. 28. Bill Nye Demonstrates Frames of Reference <ul><li>Video </li></ul>
28. 29. Frames of Reference <ul><li>Imagine that you are on a train stopped at the platform. You begin to walk toward the front of the train at 1 m/s. </li></ul>
29. 30. Frames of Reference <ul><li>Everyone on the train will agree you are moving at 1 m/s toward the front of the train. That is your speed relative to the train. </li></ul><ul><li>Everyone looking into the train from the platform will also agree that you are moving 1 m/s toward the front of the train. That is your speed relative to the platform. </li></ul>
30. 31. Frames of Reference <ul><li>Imagine you are on the same train, but now the train is moving past the platform at 8 m/s. You begin to walk toward the front of the train at 1 m/s. </li></ul>
31. 32. Frames of Reference <ul><li>Everyone on the train would agree that you are moving at 1 m/s toward the front of the train. This is your speed relative to the train. </li></ul><ul><li>Everyone looking into the train from the platform will say you are moving at 9 m/s. Why? </li></ul>